The present invention concerns a procedure for the continuous deposition of a zinc coating on a ferrous product by immersion in a bath of molten metal. Its aim is essentially to prolong the life of steel products, such as wires, tubes, and profiles or strips of steel. It applies to the method of particular interest in the case of coating in a bath of zinc containing aluminum.
It is known that the coating of steel products requires a pretreatment of the surface, for the purpose of assuring adherence, continuity and uniformity of the coating.
For certain of the aforesaid products, notably strips and wire, the preparation of the surface prior to immersion in the molten bath may consist of a thermal treatment in a reducing atmosphere. This avoids or removes all traces of oxygen or oxides on the surface of the product, which may then be immersed directly into the coating bath. Such a thermal treatment requires, however, expensive installations, and as a consequence large investments, which are not justified except in the case of important tonnages.
It is equally well known that, in the coating methods by means of dry fluxing, the preparation of the surface of the product generally comprises degreasing followed by rinsing, an acid cleaning also followed by rinsing, and a final dry fluxing. The product thus prepared is then immersed in the bath of molten metal, for example, zinc.
In this sequence of operations, the flux plays an important and multiple role. It serves especially to protect the cleaned and rinsed surface against all risk of reoxidation before introduction of the product into the zinc bath. It likewise allows the elimination of iron salts from being able to exist on the surface in spite of the rinsing performed after the cleaning, and thereby avoids their incorporation in the zinc bath. This operation is effected, in the classical way, by immersing the product in a flux consisting of an aqueous solution of zinc and ammonium chlorides.
This flux, employed for many years, plays its role of protection in a satisfactory manner as long as the coating to be deposited later is composed of practically pure zinc.
Recently, new types of zinc-aluminum coatings have been developed. These new coatings offer, depending on their composition, improved properties of ductility and adherence. These properties are very interesting, for example in the case of pieces undergoing deformation, because this deformation does not cause cracking nor detachment of the coating. Also, the ability of this zinc-aluminum coating to resist corrosion and to receive a paint film is better than in the case of a classical zinc coating.
These improved properties, due to the presence of aluminum in the coating, are particularly noted in the case of an alloy recently developed which contains, in addition to zinc, from about 3 to 15%, and preferably about 5%, aluminum and trace quantities of mischmetal. Certain of these properties are likewise present when the coating contains more aluminum, for example, up to 70% aluminum.
Nevertheless, it has been ascertained that the application of the new type of coating on a surface prepared with the aid of a traditional flux may lead to various defects in the coating. In particular, some areas of the surface may not be covered, or not covered in a sufficient manner, or the coating may show black spots or even craters which give the surface an unacceptable finish.
It appears that these defects may be due to the formation of volatile compounds, notably AlCl.sub.3, resulting from the reaction of the coating alloys with the constituents of the flux.
The deposition of the new zinc-aluminum coatings there pose a problem which, to the knowledge of the applicant, has not been resolved in a satisfactory manner up to the present time.
For example, a process is known from Belgian Patent BE-A-No. 897.788, comprising a preliminary immersion step in a bath consisting essentially of zinc and containing at least 0.5% aluminum, followed by a second immersion step in a zinc bath containing from 3 to 15% aluminum. This procedure, in which the first immersion step is preceded by a traditional fluxing, provides a coating of good quality and provides good corrosion resistance. However, it presents certain inconveniences, notably the necessity of providing and heating two coating baths, and the difficulty of controlling the composition of the Zn-Al bath, due on the one hand to the addition of zinc coming from the coated product in the first stage, and on the other hand, to the consumption of aluminum by diffusion in the Fe-Zn intermetallic layer formed during the second stage.
The present invention has for its object a process permitting the remedy of these inconveniences and the formation, by immersion in a single coating bath, of a zinc-aluminum coating which does not produce the aforesaid defects.
The procedure which is the object of the present invention is based on the discovery that the aforesaid defects are not apparent in the zinc-aluminum coatings when the flux is not deposited directly on the bare surface of the ferrous product. Rather, the present invention includes a process of continuously depositing a zinc-aluminum coating on a ferrous product, in which the said product is subjected to the operations of cleaning, rinsing, and immersion in a zinc-aluminum bath, and which is characterized by the fact that after the cleaning and rinsing operations, the aforesaid product is immersed in an aqueous solution containing a composition of at least one metal, from which solution is deposited by electrolytic means a thin layer of said metal on the surface of the product, and by the fact that after exiting the said solution, the product is dried and then immersed in the zinc-aluminum bath.
The composition of the electrolytic solution used in the process of the invention plays an important role in that it affects the quality of the final coating. According to the invention, the electrolytic solution not only enables the product to be pre-coated by electrolytic deposition, but this solution also provides fluxing properties.
In the circumstances, the applicant proposes to designate this solution by the name "Electrofluxing Solution", in order to express simultaneously the aspect of electrolysis and the aspect of fluxing. The layer of metal deposited by electrolysis is itself covered by a film having the composition of the electrofluxing solution, which film acts as a protective flux as the ferrous product is transferred from the electrolytic ("Electrofluxing") solution to the zinc-aluminum bath. In this regard, it is preferred that the electrolytic salt which provides the zinc ions for electrodeposition (e.g. ZnCl.sub.2) also provides fluxing properties. Thus in its preferred embodiment, the electrofluxing solution contains at least one compound which functions both as an electrolytic salt and a flux.
The electrofluxing solution understandably must be compatible with the composition of the zinc-aluminum bath. According to one aspect of the process, an electrofluxing solution is used containing a chloride of the metal to be deposited (e.g. ZnCl.sub.2), together with at least one other chloride or fluoride. Such a solution might contain from 100 to 700 g/l of ZnCl.sub.2, from 5 to 100 g/l of at least one chloride such as NaCl, KCl, CaCl.sub.2, and from 1 to 10 g/l of at least one fluoride such as NaF, HF, KF.
The content of ZnCl.sub.2 preferably is at least 100 g/l, in order to assure a sufficient supply of zinc for the electrolytic deposition; however, it is preferred that it not exceed 700 g/l so as not to produce too great a thickness in the deposit.
The role of chlorides (NaCl, KCl, CaCl.sub.2 . . . ) is to increase the mobility of the zinc ions and thereby to improve the efficiency of the deposition reaction. The fluorides (NaF, KF, HF . . . ), present at the preferred concentrations, increase the cleaning power of the electrofluxing solution.